On the original question of whether 1D&D will create a significant divide, when I started playing, there was already a divide: D&D and AD&D. And then there was D&D, AD&D, and AD&D 2nd Ed. However, this was not really a problem, because while they games were all distinct (probably even more distinct than 1D&D will be from 5e), they were largely the same game. We all spoke the same language, as it were, and content from one version of the game could be pretty easily used with another. There was continuity.
After WotC took over, they spent the next 14 years pursuing a strategy of reinventing the game in order to chase new players. There was no continuity at all, let alone rough compatibility. Those who did want to get onboard with the latest edition were left in the cold.
In as much as 1D&D will create a divide, I think it will look more like the TSR-era. Some people may not make the jump to the newest version of the rules, but there will enough continuity that they won't be left entirely in the cold. New players will mine old 5e stuff for the 1D&D games, and vice-versa. Internally, you'll have your 1D&Ders, your 5e'ers, your Level Uppers, your Black Flaggers, but functionally, we'll be united as one big 5e community.